add error in case of improper inputs

tests/test_ensembles.py

@@ -223,6 +223,8 @@ def test_common_attrs_only(self, common_attrs_only):
 
     def test_errors(self):
         ens = self.make_ensemble(10)
+
+        # Warning if the statistic does not support weighting
         weights = xr.DataArray([0] * 5 + [1] * 5, dims="realization")
         with pytest.warns(UserWarning, match="Weighting is not supported"):
             with pytest.raises(
@@ -231,6 +233,8 @@ def test_errors(self):
                 xs.ensemble_stats(
                     ens, statistics={"kkz_reduce_ensemble": None}, weights=weights
                 )
+
+        # Error if you try to use a relative delta with a reference dataset
         for e in ens:
             e["tg_mean"].attrs["delta_kind"] = "rel."
         ref = weights
@@ -248,6 +252,22 @@ def test_errors(self):
                 },
             )
 
+        # Error if you try to use a robustness_fractions with a reference dataset, but also specify other statistics
+        with pytest.raises(
+            ValueError, match="The input requirements for 'robustness_fractions'"
+        ):
+            xs.ensemble_stats(
+                ens,
+                statistics={
+                    "robustness_fractions": {
+                        "test": "threshold",
+                        "abs_thresh": 2.5,
+                        "ref": ref,
+                    },
+                    "ensemble_mean_std_max_min": None,
+                },
+            )
+
 
 class TestGenerateWeights:
     @staticmethod

xscen/ensembles.py

@@ -150,6 +150,10 @@ def ensemble_stats(  # noqa: C901
             # FIXME: This can be removed once change_significance is removed.
             #  It's here because the 'ref' default was removed for change_significance in xclim 0.47.
             stats_kwargs.setdefault("ref", None)
+            if (stats_kwargs.get("ref") is not None) and len(statistics.keys()) > 1:
+                raise ValueError(
+                    f"The input requirements for '{stat}' when 'ref' is specified are not compatible with other statistics."
+                )
 
             # These statistics only work on DataArrays
             for v in ens.data_vars: